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Onefin Catshark (Pentanchus profundicolus)

The Onefin Catshark has perplexed researchers since its discovery in 1909. It is the only known five-gilled shark with one dorsal fin, so scientists at the time believed they found the missing link between Hexanchiformes (single dorsal-finned six- and seven-gilled sharks) and the rest of the shark world, which all have five gills. This theory was controversial however, since the only two specimens are distinctly catshark-like and bear no resemblance to Hexanchiformes with the exception of their single dorsal fin.

The discovery of Pentanchus profundicolus changed the scientific understanding of scyliorhinid sharks. Known from a single specimen for 90 years, the holotype was badly damaged and in several pieces. A second specimen discovered in June 1985 shed some light on this enigmatic species. X-rays have confirmed that the specimens are not merely damaged; rather, having one dorsal fin is the natural form of the Onefin Catshark.

The general consensus in the shark world is that the Onefin Catshark is not the missing link between Hexanchiformes and Scyliorhinid sharks. Where does it belong in the shark family tree? Is it an existing ancestor of two-finned catsharks? Or is a second dorsal fin all that important? The Onefin Catshark is a reminder that the shark world is wide with mystery.